Solder is a class of materials used to provide electrical, mechanical and thermal interconnections, and different solder alloys are marketed for different applications such as jewelry, plumbing and electronics. The most common solders are composed of lead and tin, with solders containing about 60% tin and about 40% lead being the principal type of solders used in soldering electronic assemblies.
One of the disadvantages arising from solders is their contribution to environmental pollution. Environmental pollution is caused by lead precipitation from solder joints in soldered materials that are discarded. For this reason, federal legislation has been introduced to reduce lead content in commercial solders.
A second disadvantage arising from solders is their poor mechanical properties due to the metallurgical structure of solder. Solder grains are bonded to base metals by intermetallic compounds. External loading on this matrix generates inter granular deformation and fracture both in the bulk solder and at the intermetallic interface. Improving the mechanical properties of solder would broaden its use into new applications, reduce manufacturing costs by eliminating the need for scientifically unfounded process controls, and reduce maintenance by extending product life cycles.
Poor mechanical properties were resolved in the thermoplastics industry by using composites wherein glass fillers in chopped fiber or woven cloth form were added to the resin. The purpose of adding such fillers was to interconnect sliding polymer chains and to disperse propagating cracks. A similar principal has been proposed for solder. For instance, a solder-matrix composite of tin-lead solder containing continuous unidirectional copper-plated carbon fibers to increase the strength of the solder is disclosed in Ho and Chung, Carbon Fiber Reinforced Tin-Lead Alloy as a Low Thermal Expansion Solder Preform, J. Mater. Res., Vol. 5, No. 6, June 1990, pp. 1266-70. This composite solder, however, is not practical for mass production applications such as electronics. In addition, the fiber composite was designed for low thermal expansion, and fibers would be unsuitable for penetrating crevice areas of solder joints.
The use of molybdenum particles as a filler in solder-matrix composites has also been attempted. Such attempts were unsuccessful because their distribution in the solder became nonuniform after remelting and solidification of the solder in the composite. Thus, molybdenum particles are not suitable for use in composite solders. See Ho and Chung, supra, at 1266.